Rotating machines generally include bearings enabling the rotating portion of the machine to rotate relative to stationary portions of the machine without excessive frictional forces being exerted on the rotating portion. One class of bearings is characterized by rolling elements, such as balls or rollers, confined in a raceway defined between inner and outer ring-shaped races. The rotating portion of the machine is fixed to one of the races, and as it rotates about the axis defined by the bearing, the rolling elements roll along the surfaces of the races. In many rotating machines, bearings of this type are capable of providing relatively long bearing life and meeting the desired performance characteristics.
Fluid film bearings define another class of bearings. In a fluid film bearing, a film of fluid is disposed in a space defined between the rotating and stationary bearing elements. Thus, instead of rolling elements, fluid is used for supporting the rotating bearing element and preventing contact between the rotating bearing element and the stationary bearing element. Because there is no metal-to-metal contact within the bearing, galling and other types of physical degradation of the bearing caused by metal-metal contact are reduced or eliminated. Fluid film bearings in many cases can also offer improved damping compared to ball or roller bearings, and the damping characteristics can be tuned by suitable selection of the clearance between the rotating and stationary bearing components, surface finish of the bearing components, and other factors, in order to provide the desired performance characteristics. Thus, fluid film bearings can offer improved performance relative to conventional ball or roller bearings, especially for very high-speed machinery such as turbopumps or the like.
Annular fluid film bearings are used in some types of rotating machines. An annular bearing has a relatively small radial clearance between the rotating element and the stationary element, and hydrostatic pressure of the fluid within the clearance supports the rotating element in the radial direction and enables the rotating element to rotate. The fluid in the annular clearance provides stiffness and damping for the bearing. In some types of high-performance machines, the rotating component frequently must operate at a speed that is above the first and second shaft critical speeds. In order to avoid high vibrations when passing through the critical speeds, the rotating component, and therefore the bearings, should have a high degree of direct damping and stiffness and a low cross-coupled stiffness.